1. Field of the Invention
This invention relates to a dual mode drum brake and, more specifically, to a dual mode drum brake which functions as a leading-trailing (LT) type brake device during the service brake operation and which functions as a duo servo (DS) type brake device during the parking brake operation.
2. Description of the Related Art
Conventional dual mode drum brakes are, for example, disclosed in Australian patent number AU-B1-53 491/79, U.S. Pat. No. 5,275,260, and U.S. Pat. No. 6,003,645. Because the basic structure of these conventional dual mode drum brakes is adequately disclosed in the aforementioned publications and because they have identical structures and operation, the typical compositions are explained hereunder with reference to FIG. 6 only.
A drum brake comprises a back plate 10 as a stationary member; a pair of brake shoes which includes a first brake shoe 11 and a second brake shoe 12 that are facing each other and are mounted on the back plate 10; a service brake actuator (i.e., a wheel cylinder actuator 15 in this figure) positioned between upper adjacent ends (or one adjacent ends) of two brake shoes 11 and 12; an anchor block 16 positioned between lower adjacent ends (or the other adjacent ends) of two brake shoes 11 and 12; a shoe clearance adjustment device 18 positioned adjacent to the wheel cylinder actuator 15 and extended between the two brake shoes 11 and 12; a parking brake actuator 23 positioned adjacent to the anchor block 16; and a pivot lever 22 pivotally supported on the first brake shoe 11 and positioned between both ends of the first brake shoe 11, an upper (or one) side and a lower (or the other) side of which is functionally engaging with the shoe clearance adjustment device 18 and the parking brake actuator 23, respectively. Shoe return springs (not shown in the figure) urging the first and the second brake shoes 11 and 12 toward each other, are extended between two adjacent upper sides of the brake shoes 11, 12 and between two adjacent lower sides of the brake shoes 11, 12 respectively.
Brake operation of the above-described structure will be explained next, and service brake operation will be explained first.
If the wheel cylinder actuator 15 is pressurized, both brake shoes 11 and 12 move and spread apart from each other around a point of abutment with the anchor block 16, thereby causing frictional engagement between brake shoes 11, 12 and a brake drum 5.
Parking brake operation will be explained next.
If a brake lever 24, a component of the parking brake actuator 23, is pulled in the direction of an arrow X, an operational force generated thereat is transmitted to a strut 25, also a component of the parking brake actuator 23, a pivot lever 22, and a shoe clearance adjustment device 18, thereby moving the second brake shoe 12 to cause a frictional engagement with the brake drum 5.
Then, the pivot lever 22 rotates counterclockwise around a point of abutment with the shoe clearance adjustment device 18, and the first brake shoe 11 is thrust in the direction of an arrow Y via a pivot section of the pivot lever 22 causing a frictional engagement with the brake drum 5.
Also, a reaction force of the brake lever 24 in a direction of an arrow Z acts on lower end of the second brake shoe 12. If a rotational force in the direction of an arrow R acts on the brake drum 5 in the situation where a vehicle is on a slope, a frictional force of the first brake shoe (primary brake shoe) 11 is transmitted to the second brake shoe (secondary brake shoe) 12 via the shoe clearance adjustment device 18, which generates a braking force as a duo servo brake.
If a rotational force in an opposite direction of the arrow R acts on the brake drum 5, a frictional force of the second brake shoe (primary brake shoe) 12 is transmitted to the first brake shoe (secondary brake shoe) 11 via the shoe clearance adjustment device 18, which generates a braking force as a duo servo brake similar to the above-described duo servo brake.
The previous dual mode drum brake with the above-described structure had the following points to be improved.
When a vehicle is stopped on a slope, the operator of the vehicle will apply the service brake using the service brake pedal. Thereafter, the parking brake lever, usually a side brake lever, is pulled and locked to maintain the vehicle's stopped position and a pressing force on the brake pedal is decreased or the brake pedal is released.
The position of the contact point between an upper side of the pivot lever 22 and the shoe clearance adjustment device 18 varies because the position is determined by the dimensional preciseness of the pivot lever 22 and the first brake shoe 11.
For example, if the shoe clearance adjustment device 18 contacts the pivot lever 22 and a clearance exists between the shoe clearance adjustment device 18 and the brake shoe 11, the brake drum 5 rotates in either direction to carry the primary brake shoe 11 or 12, i.e., the brake shoe disposed at a primary side of a drum rotational direction, until the shoe clearance adjustment device 18 and the first brake shoe 11 contact each other. Therefore, the piston of the wheel cylinder actuator 15 is forced back by an amount that corresponds to the range of movement of the primary brake shoe 11 or 12.
As a result, the brake pedal is forced back, thereby giving the operator an uncomfortable feeling and also increasing the pedal stroke distance when operating the service brake immediately after above-operation.
During the parking brake operation, the brake drum 5 rotates in the opposite direction of the arrow R. The brake lever 24 is moved together with the second brake shoe 12 as the primary brake shoe. In this situation, a brake cable pulling the brake lever 24 is loosened, and braking effectiveness is suddenly decreased. If the operator reduces the pressure on a brake pedal or releases the brake pedal then this could cause a potentially dangerous situation and the operator to may need to take additional, immediate braking action.
If the shoe clearance adjustment device 18 contacts the first brake shoe 11 and clearance exits between the shoe clearance adjustment device 18 and the pivot lever 22, a stroke loss of the parking brake lever increases by an amount that corresponds to the clearance, thereby giving the operator an uncomfortable feeling when operating the parking brake
If the pivot lever 22 is made in one piece by press, a contact surface of the pivot lever 22 with the shoe clearance adjustment device 18 becomes a fractured rough surface which tends to cause permanent deformation thereof and may gradually vary the point of contact with the shoe clearance adjustment device 18.
If an automatic shoe clearance adjustment device that automatically adjusts the clearance between the linings and the brake drum 5 does not exists, then when the linings of the brake shoes 11 and 12 are worn out, the brake stroke gradually increases, thereby causing the operator an uncomfortable feeling when operating the brake.
This invention was made to improve the above-points and also was designed to provide a dual mode drum brake device that provides less stroke loss during brake operation. This thereby constantly gives a stable parking brake effectiveness and furthermore gives a comfortable feeling for the operator when operating the brake.